valid array indexn. (of an array) a fixnum suitable for use as one of possibly several indices needed to name an element of the array according to a multi-dimensional Cartesian coordinate system. Such a fixnum must be greater than or equal to zero, and must be less than the corresponding dimension[1] of the array. (Unless otherwise explicitly specified, the phrase ``a list of valid array indices'' further implies that the length of the list must be the same as the rank of the array.) ``For a 2 by 3 array, valid array indices for the first dimension are 0 and 1, and valid array indices for the second dimension are 0, 1 and 2.''